Mark-recapture methods for monitoring Sonoran populations of the desert tortoise (Gopherus agassizii)

Abstract

I determined reliable methods for monitoring Sonoran populations of the desert tortoise. Tortoises were significantly clumped (p < 0.001) on three 1-mi² plots in Arizona, but were not significantly different from random (p > 0.05) on a 1-km² plot, supporting the use of smaller plots in mark-recapture population studies. Simulations showed that Program CAPTURE's Jackknife and Darroch estimators are robust to variations in capturability, which confound most commonly used estimators (e.g., Lincoln-Petersen). Mean capturability determines which estimator is most appropriate for a given population. These methods were applied to data from several tortoise populations. CAPTURE's Jackknife method estimated 70 tortoises/km² in a Mazatzal Mountain, Arizona, population surveyed during 1992. Density was corrected with the mean maximum distance moved method. Regression of CAPTURE estimates indicated two separate populations were stable or increasing from 1990 to 1992, while a third declined. Program JOLLY estimated high survivorship for these three populations (87-100%), but recruitment was lowest for the decreasing population (0-17 tortoises/year).